The present invention relates to a machine tool having a spindle rotating in a headstock and, further, comprising a gripper for transferring a tool holder from an uncoupled position into a ready-to-couple position within the spindle. A coupling mechanism is provided for further transferring the tool holder from the ready-to-couple position to a coupled position within the spindle in which the tool holder can be rigidly connected to the spindle for rotation therewith. In order to control the transferring steps, a sensor is provided for detecting whether the tool holder is in its coupled position.
Such a machine tool is known from U.S. Pat. No. 5,263,918.
In the known machine tool a tool holder with clamped tool is inserted into the spindle by means of a gripper arm. The tool holder is rotatably mounted in the gripper arm under friction.
In order to make a torsionally rigid connection between the tool holder and spindle the surface of the spindle facing the tool holder is provided with a spring-borne catch. The opposite face of the tool holder, which is already aligned coaxial to the spindle, is provided with two grooves which lie on two different radii so that the catch can only engage in one of the grooves. When the tool holder is inserted into the rotating spindle that catch does not usually engage exactly in one groove but makes contact with the otherwise smooth face of the tool holder so that the catch is pressed back into the spindle against the force of the spring. A relative movement between the tool holder and catch now occurs through the pulling effect of the catch and on account of the friction of the tool holder in the gripper so that this slides along the face of the tool holder and finally "locates" the corresponding groove.
The catch is assigned a signal rod which is borne in such a way that it can slide along the spindle and which follows the relative movements of the catch to the spindle. Moreover, a concentric signal ring is provided on the spindle which follows the movements of the signal rod so that this is in an initial axial position when the tool holder is in a couplable position and assumes an second axial position when the tool holder has been coupled. Furthermore, a fixed transducer is provided which interacts with the signal ring in such a way that it can differentiate between the two different axial positions.
The transducer consequently initially signals that the catch is located in its lower axial position during insertion of the tool holder in the spindle, which means that there is no tool holder in the spindle. During insertion of the tool holder in the spindle the signal ring is pushed upwards so that the transducer signals that a tool holder is in a couplable position. When the catch has now engaged in "its" groove" the signal ring falls back into its lower position, which is detected by the transducer. This sequence of signals from the transducer is processed by a sequence controller which runs the spindle up to operating speed immediately after the coupled position has been assumed.
The tool is thus coupled very quickly through a monitoring of the catch.
On account of the fact that only one of the two grooves lies on the correct diameter, so that the catch can only engage in this groove, the tool is always aligned "correctly" to the spindle. This orientation is important, e.g. during work with lathe tools, which must be correctly oriented when retracted from a bore hole to prevent scoring.
One disadvantage of the known machine tool is that the tool change time depends largely on the random position at which the gripper inserts the tool holder into the spindle. For example, the catch can engage in the correct groove after a veryshort relative rotation between the spindle and tool holder, though this relative movement can also be up to almost 360.degree.. The relative speed between the tool holder and spindle can be low, particularly in the case of a tool holder which rotates almost unbraked in the gripper, so that the tool change can take a long time.